Vehicles with parallel hybrid drive systems have an internal combustion engine as well as at least one electric motor that drives at least one vehicle wheel using the same or another drive train of the internal combustion engine. As long as the electric motor can also be operated as a generator, then it is possible for the electric motor in almost every point of the vehicle operating map to provide motoric support or regenerative additional loading by the electric motor and to decouple from the internal combustion engine operation to a certain extent.
This is accompanied by a change in the exhaust gas quality, particularly with respect to the exhaust gas mass flow, the exhaust gas temperature, as well as the composition of pollutants. Such a change in the exhaust gas quality affects the functioning and the efficiency of a catalytic converter system arranged downstream of the internal combustion engine. The patent application EP 1 182 074 A2 describes a method in which the operation of the electric motor as a generator is used to increase the load of the internal combustion engine in case of a catalytic converter temperature that lies below a characteristic light-off temperature. This helps achieve an increased exhaust gas temperature and a faster activation of the catalytic converter system after a cold start. It is further suggested in this patent application to bring about an unloading of the internal combustion engine by the motoric operation of the electric motor in case of a motor or catalytic converter at operating state temperature.
This operating mode is very unilaterally designed for shortening the light-off phase since it does not take into consideration that the increase in the load of the internal combustion engine also involves a considerable increase in the untreated exhaust gas emission, especially the untreated exhaust gas emissions of hydrocarbons. Thus the reduction of the emission of pollutants, particularly of hydrocarbons, by shortening the light-off phase of the catalytic converter system can be overcompensated by the increase in the untreated exhaust gas emissions and can sometimes lead to increased total emissions downstream of the catalytic converter system (tail pipe emissions).
Furthermore, a hybrid vehicle is known from the patent application DE 100 41 535 A1 in which a generator generates electrical energy in case of a non-activated catalytic converter and thus increases the load of the internal combustion engine. This increases the temperature of the internal combustion engine, the temperature of the exhaust gas released by the internal combustion engine or the temperature of the coolant of the internal combustion engine for an accelerated activation of the catalytic converter.